1. Field of the Invention
The present invention is directed to apparatus, systems and methods for expanding the ability of existing communication transmission systems to carry information, including but not limited to television broadcast, cable television, microwave systems, closed circuit television, FM broadcast and other closed circuit and broadcast systems.
2. Background
In 1941, the Federal Communications Commission (“FCC”) adopted standards for commercial television broadcasting in the United States. Named after the committee that created it, the National Television Systems Committee (“NTSC”) standard is the approved method for over-the-air transmission of television signals in the U.S. This television technology is an analog system, wherein the picture component is transmitted in a vestigial sideband modulation format on the visual carrier and the sound component is transmitted as frequency modulation on a separate sound carrier. In 1954, the National Television Systems Committee “compatibly” extended the NTSC system to include color information by increasing the utilization of the 6 MHz spectrum occupied by the television channel.
The NTSC standard is termed “analog” because the picture and sound information can take on any value between the minimum and maximum values. An infinite number of values are possible. The picture information is related to the strength of the transmitted signal with black portions of the picture having the most power and white portions of the picture having the least power. Periodic pulses are included at powers greater than those used to represent black areas in the picture. These pulses provide the timing information required to synchronize the transmitter and the receiver so that the picture is shown correctly on the screen. The horizontal synchronization pulses coordinate the left-to-right placement of images on the screen while the vertical synchronization pulses coordinate the top-to-bottom placement of the picture.
The cathode ray tube (“CRT”) was the original display device used in high volume production television receivers. A CRT uses an electron beam to stimulate a phosphor coating on the inside face of a vacuum picture tube. The electron beam scans the tube horizontally and vertically to display a complete image. The strength of the electron beam is inversely proportional to the strength of the television transmitter power and regulates the amount of brightness in the picture. The deflection of the electron beam can be accomplished by electrostatic forces or magnetic forces. Most television display devices used magnetic deflection. Magnetic deflection requires time to move the electron beam back to the left side of the screen after completing a line. During this time, the electron beam must be turned off or blanked to prevent unintended stimulation of the phosphor screen and the resulting interfering light. The period of time during which the electron bean is turned off is called the “horizontal blanking interval.” When the electron beam reaches the bottom of the screen, it must be returned to the top of the screen to continue the process of making pictures. Just as in the horizontal case, the electron beam must be blanked to prevent disturbing light patterns on the screen. This period is called the Vertical Blanking Interval (“VBI”). The VBI is much longer than the horizontal blanking interval. The combination of the two blanking intervals constitutes approximately twenty-five percent of the total scanning time. This time may not be used to convey analog pictures.
The scanned picture area is termed a “raster”. The raster consists of two half-pictures called fields. Two fields make up a complete picture which is called a frame. One field has the even scan lines while the other field has the odd scan lines. The fields are displayed at a rate of sixty fields per second. This technique of using two fields is called “interlace” and reduces the flicker of the image while conserving bandwidth.
The Vertical Blanking Interval
It was determined that other uses can be made of the electron blanking time. For example, the VBI may be used to carry analog test signals that measure the transmission characteristics from the signal source to intermediate points along its path to the final point of use. The VBI may also be used to carry analog signals representing digital data. The data signals may be of two or more levels which are resolved into data bits by appropriate circuits. Since the “digital” signals are of just a few discrete levels, the data detection circuits can discriminate against significant amounts of noise, distortion, and interference. This makes these data signals more robust than the analog Visual signal itself for most forms of interference.
The United States first attempted to use the VBI for ancillary purposes in 1970 when the National Bureau of Standards (“NBS”) proposed to use the VBI for the distribution of precise time information nationwide. The ABC television network was a partner in that effort. While this initiative did not result in a service, ABC recommended a captioning service for the hearing impaired.
The First National Conference on Television for the Hearing Impaired met in Nashville, Tenn. in 1971. The NBS and ABC subsequently demonstrated a captioning service at Gallauded College in early 1972. In 1973, the engineering department of the Public Broadcasting System (“PBS”) initiated development of a captioning service to be funded by the department of Health, Education and Welfare (“HEW”). As a result of this work, the FCC reserved line 21 of field one of the NTSC television signal for the transmission of closed captions in the United States in 1976. In 1979, the National Captioning Institute (“NCI”) was founded to caption programming and to further the cause of captioning. In the early 1980s, Sears Roebuck stores carried a captioning decoder in set top box configuration selling for about $250. In 1989, NCI contracted for ITT Semiconductor Corporation to develop a cost-effective caption decoder microchip for use in television receivers. In 1990, Congress passed the Television Decoder Circuitry Act mandating that new television receivers of thirteen-inch diagonal display measure or greater include caption decoding circuits after Jul. 1, 1993. Approximately twenty million television receivers per year are covered by this requirement. In 1992, NCI, the FCC, and the Electronic Industries Association (“EIA”) developed captioning technical standards. The 1996 Telecommunications Act requires the FCC to promulgate rules requiring closed captioning on Visual programming but allowing exemptions for programming that would suffer an “undue burden”.
The Closed Captioning (“CC”) system is called “closed” because it is turned “on” or “off” depending on the user of the television receiver. Those without hearing impairments and those who understand the spoken words need not be disturbed by text on their screens. The CC system supplies data to appropriate digital and analog circuits that place carefully timed text on the television screen to allow the hearing impaired to read a description of the conversation taking place and have indications of other relevant sounds. Moreover, those who cannot understand the spoken words may have text translated into their native language so that they may follow the program. The CC system uses very low speed data in order to minimize the impact of transmission path problems such as reflections and interfering signals. The data rate for the CC systems is 503,500 bits per second of binary (two level) data. This data rate is expressed as 503.5 Kilobits per second (“kb/s”). This data rate allows only two eight-bit characters to be transmitted per VBI line. If only field one is used, about two lines per second may be displayed. This rate yields 480 bps or 3,600 characters per minute. If the average word is five characters long and is followed by a space, then 600 words can be conveyed per minute. The rest of the VBI line is occupied with both a burst of seven sine wave cycles of 503.5 kHz clock run-in and a unique “start bits” pattern placed at the beginning of the line. These signals synchronize the detector circuitry. Since FCC rules protect only Line 21 for captioning, the rate of transmission is slow, but adequate for the purpose. The on-screen CC display consists of a maximum of fifteen rows of thirty-two characters each. The captions usually appear only on rows one through four and rows twelve through fifteen. The middle rows are usually transparent to show the action. A text mode provides scrolling text. Further details can be found as part of the EIA standard number EIA-608 which is incorporated by reference herein. The CC system was designed at a time when electronic circuits for the correction of transmission deficiencies were very expensive. Since physically-challenged consumers were the target market for CC systems, the industry sought to minimize the cost of the equipment. An extremely conservative data rate facilitates inexpensive circuits by minimizing the technical challenge.
The closed captioning signal carries four components: two captioning “channels” and two text channels. The first captioning channel is synchronized to the Visual programming so that the words carefully match the video. The second captioning channel is not synchronized.
The EIA filed a petition with the FCC to expand the captioning standard EIA-608, to allow use of line 21, field 2. This would add two additional captioning channels and two additional text channels. A fifth channel has been added to carry Extended Data Services (“EDS”). EDS will carry a wide variety of additional information. Precise time information will be transmitted to set VCR clocks (and other clocks as well). The channel's name and call letters are included along with current program information such as title, length, rating, elapsed time, types of Aural services and captioning services and intended aspect ratio. EDS also includes the data for the “V-chip” (Violent programming advisory) which is intended to facilitate parental control of children's access to programming that parents might deem objectionable. Public service announcements such as weather and emergency advisories may also be transmitted. Cable system channel layout information will be provided so that the channel number indicator can use the more familiar channel identification number rather than the number associated with the frequency utilized. This facility will bring the same “channel mapping” benefits subscribers have enjoyed in their cable set top terminals to consumer electronic products.
A subsequent VBI data transmission system, “Teletext,” was invented to provide ancillary services to television users. The Teletext system can display up to twenty-four rows of forty characters (but a specification of twenty rows was selected for the U.S.) on the television screen. Teletext quickly evolved into a transmission system for more complex data, including the “downloading” of software to computers. It was introduced at a time when electronics were still relatively expensive, but less expensive than at the time of introduction of the CC system.
Teletext is a more aggressive form of data transmission which has been successful in Europe, but has failed to enjoy commercialization in the U.S. Teletext originated in Great Britain with experimental transmission commencing in 1972. The British Broadcasting Corporation (“BBC”) branded their Teletext service “Ceefax” while the Independent Broadcast Authority (“IBA”) called their service “Oracle”. France developed a packet-based Teletext system called “Antiope” based on a transmission system called “Didon.” Later, Canada developed another system called “Telidon” which featured higher resolution graphics. The Japanese system, “Captain,” featured “photographic coding” to accommodate the Chinese Kanji characters and the Japanese Kana character set.
Teletext has had difficulties in the U.S. for a number of reasons. The principal reason for the problem was the failure to find a successful commercialization strategy. Without this, the system could not be supported. Additional difficulties included the high cost of memory at the time of implementation. While a Teletext page requires only about a kilobyte of storage, that small amount of memory was considered too expensive at the time of development. Further problems centered around the quality of the graphics. The less expensive World System Teletext (“WST”), based on the British approach, had crude “Lego-style” graphics in its basic form. The other contender, the North America Presentation Layer Protocol System (“NAPLPS”) used a higher resolution graphics system that painstakingly painted itself on the screen, resulting in excessively long delays that tried the patience of the average consumer. Still another complication was the FCC's 1983 decision to allow two standards, with the marketplace deciding the eventual winner. One of the standards was WST, the other was the NAPLPS evolution of Antiope, Telidon, and efforts by AT&T. Reliability of data reception was the final problem. In a test in the Bay area of San Francisco, only about twenty-five percent of installations of the NAPLPS system were trouble-free. The remainder suffered from various degrees of multi-path impairment. The more robust WST system was not tested in that environment.
Both U.S. Teletext systems have a data rate of 5.727272 Mb/s which is 364 times the horizontal scanning rate and 8/5 of the frequency of the color subcarrier. The data signal has a Non Return to Zero (“NRZ”) binary format. The WST data line consists of eight cycles of clock run-in (sixteen bits), followed by a unique eight bit “framing code,” followed by sixteen bits of control codes and a payload of thirty-two eight-bit display words. Because forty characters are displayed in a Teletext row of text and only thirty-two are transmitted per scan line, the additional eight characters from four rows of text are put on an additional supplementary scan line. Thus five scan lines are required to convey four rows of text. Twenty rows would require five additional supplementary scan lines. A page format of forty characters by twenty rows with an additional “header row” of only thirty-two characters, requires twenty six field lines per page of WST Teletext. The payload of 256 bits per line allocated means that if one VBI line in each field is allocated, a data rate of 256×2×30=15,360 bps is obtained. Eleven lines of VBI are possible (Line 21 is reserved for captions and the first nine lines form the vertical synchronization pulses) yielding a maximum of 153 kb/s of data for full VBI utilization.
The WST system maps the data location in the VBI line to memory locations and to screen locations and always stores data in the same memory place. This allows for a very simple error protection scheme. Since the instructions in the header are Hamming Code protected, a measure of the quality of the received signal is obtained. If the signal is of low quality, it is not stored in memory. Only good quality data is stored. As a result, good data can be accumulated from repetitions of the page until a good page of data is built up. It is also possible to use a “voting” approach to obtain very robust transmission.
The fundamental difference between the WST and the evolving set of Antiope, Telidon, and NAPLPS systems is that the latter three systems all used a packet structure. They have been characterized as asynchronous because there is no mapping between the transmission scheme and memory and screen locations.
PBS has developed a packetized data delivery system based on Teletext called the “PBS National Datacast Network”. The standard Teletext data rate of 5.72 Mb/s is used yielding 9600 baud per VBI line allocated per field. The Datacast network distributes the same signal nationally. The goal is to generate revenue to help support the PBS network. The Datacast signal has a wide variety of commercial applications. Currently, the StarSight Electronic Program Guide (“EPG”) signal is distributed via PBS.
With the advent of Teletext service, the FCC was once again (as in the case of the addition of color) forced to decide between advancing new and useful television service enhancements and new and useful communications services on the one hand and minimizing adverse effects on existing television receivers on the other hand. Certain classes of television receivers displayed the Teletext data as a series of dots arrayed diagonally near the top of the displayed picture. The FCC amended its rules on May 20, 1983 (53RR2d 1309) to permit a phased introduction of the Teletext signal to “avoid potential degradation . . . on some existing receivers”.
While CRTs remain the primary display devices in consumer electronics products, a variety of non-CRT devices are used to display pictures. Many of them are free of the constraints of retrace. However, television signals must continue to support the existing population of approximately 250 million CRT display devices owned by consumers. Thus the VBI remains a critical part of the television signal.
Vestigial Side Band Modulation
Another important characteristic of the analog NTSC television system is its Vestigial Side Band, (“VSB”), modulation scheme, described more fully below. Television channels are combined into a spectrum of signals by modulating them onto carriers of different frequencies. This makes it possible to transmit many of them simultaneously and to use frequency selective circuits to choose just one signal for processing and display. This method is called Frequency Division Multiplexing (“FDM”). When a signal is modulated onto a carrier by multiplying the base band signal with the carrier frequency, a double side band signal results. This is a consequence of the multiplication of two mathematical sine (or cosine) functions. From the mathematics of trigonometry, the multiplication of two sine (or cosine) functions yields the sum of two cosines. One of the elements of that sum has an angle equal to the sum of the angles of the multiplied cosines (sines); the other has an angle equal to the difference of the multiplied cosines. Thus:
                                          cos            ⁡                          (              A              )                                ·                      cos            ⁡                          (              B              )                                      =                                            1              2                        ⁢                          cos              ⁡                              (                                  A                  -                  B                                )                                              +                                    1              2                        ⁢                          cos              ⁡                              (                                  A                  +                  B                                )                                                                                                  sin            ⁡                          (              A              )                                ·                      sin            ⁡                          (              B              )                                      =                                            1              2                        ⁢                          cos              ⁡                              (                                  A                  -                  B                                )                                              -                                    1              2                        ⁢                          cos              ⁡                              (                                  A                  +                  B                                )                                                        
One of the sine (or cosine) functions is of fixed amplitude and fixed frequency. This frequency is much higher than the other sine (or cosine) and has significant power. It is called the “carrier” because it supports the conveyance of the information. The information includes a complex collection of other sine and cosine functions. Multiplying these functions together yields sum and difference frequencies. The multiplication process results in a version of the information placed above the carrier frequency, called the upper sideband, and its mirror image, called the lower sideband, placed below the carrier frequency. The unfortunate consequence of this is that double the bandwidth of the information signal is required. Since Visual signals have a base band bandwidth of 4.2 up to MHz, up to 8.4 MHz would be required to transmit the entire signal. The disadvantage of using this much spectrum per signal is that the total number of possible signals is more limited than in the absence of double side band signals. Since the same information is present in both sidebands, it is possible to convey all of the information with just one sideband. However, at the time the NTSC system was created, such circuitry would have had to have been implemented with many vacuum tubes. While today's electronics could easily and cost effectively build such systems into consumer electronics products, the state of development of the early television consumer electronics would have found such systems prohibitively expensive.
Double sideband signals can be recovered with simple circuits called “envelope detectors”. This is possible because the outline of the power curve of a double sideband signal follows the baseband signal exactly. A compromise was made. It was determined that if a portion—a “vestige”—of the lower sideband was included, a simple envelope detector could still be used and the distortion introduced was minimal and acceptable. The filtering required at the receiver to compensate for this was modest and affordable. This filtering results in the VSB modulation of the television signal. In NTSC the lower sideband (vestigial sideband) is truncated with a filter that results in the first 750 kHz below the visual carrier being essentially unattenuated, energy between 750 kHz and 1.25 MHz being attenuated at a prescribed rate, and the energy below 1.25 MHz being essentially abated.
All consumer television sets and radios are built upon the well-known superheterodyne receiver principle. When selecting a television or radio signal embedded in a broad spectrum of other signals, the receiver must pass the desired signal and reject all others. The receiver accomplishes this process with a frequency selective filter. The design of this filter becomes immensely more complex if the receiver is intended to select different programs at different times. The design of frequency selective filters that cover a wide range of frequencies is complex and uneconomical. An alternate approach is to design a fixed frequency filter that operates at an Intermediate Frequency (“IF”) and adjust the spectrum so that the desired signal is moved to the frequency of the fixed filter. The fixed frequency filter is called the IF filter.
The receiver moves the spectrum by multiplying it by a suitable frequency cosine (or sine) wave called the Local Oscillator (“LO”) signal. As previously discussed, this multiplication results in the creation of sum and difference frequencies, adding the entire spectrum of frequencies to the frequency of the LO and also subtracting the entire spectrum of frequencies from the LO frequency. The LO frequency is chosen so that either the sum or the difference set of frequencies pass through the IF filter. The adjustment of the LO to cause different signals to be selected is both straight forward and very cost effective.
The process of multiplication of the spectrum with the LO cosine wave can be done in any non-linear device. It is usually done in a balanced mixer that cancels out the LO frequency. This part of the receiver is usually called a “mixer” or, in older literature, the “first detector”.
Compromise “Compatible” Color Television
Yet another important characteristic of the television signal is the clustering of energy around harmonics of the scan rates. This clustering is a consequence of the redundancy in the analog image and the periodic horizontal and vertical scan rates. The clustering of energy has made it possible to interleave additional information. Interleaving was first used advantageously when the monochrome television system was extended to include color. A subcarrier at about 3.58 MHz is locked to the horizontal scan rate so that its energy is clustered at frequencies that fall between the existing energy clusters for the monochrome signal. This technique allows color television to be “compatible” with monochrome television. Compromises make this compatibility incomplete. Monochrome receivers built before the introduction of color had Visual bandwidths of up to 4.2 MHz. This allowed for very sharp black and white pictures. When color signals were introduced, these receivers suffered from “dot crawl”. The color signal was not adequately rejected by the older receiver and appeared as a moving pattern of faint, but annoying dots. This problem was overcome in later monochrome receivers by introducing a notch in the frequency response to eliminate much of the color signal. The consequence was a loss of resolution and sharpness. Alternatively, the Visual bandwidth of monochrome receivers was rolled off so that the color signals were attenuated. This too, reduced sharpness. These compromises allowed the two types of receivers, color and monochrome, to continue in production and receive the same signals. But this came at the cost of reduced performance in new monochrome receivers and degraded performance in monochrome receivers manufactured before the introduction of color receives.
This compatibility was critical to the rational introduction of color television into a market already populated with monochrome television receivers. Consumers with investments in monochrome receivers continued to access service while consumers who purchased color receivers derived more benefit from the same signals. Those who could not afford a color receiver could buy a new monochrome receiver and still have access to television. No one was disenfranchised by the technological advance to color.
Research has shown that the human visual system can see most colors based on combinations of red, green, and blue stimulation. These three signals can be algebraically combined into a signal that conveys the monochrome information, and two so-called “color difference” signals which carry the information to construct colors. The human eye is most sensitive to colors near flesh tones. Accordingly, the color television system is designed to maximize the fidelity of flesh color.
The two “color difference” signals are modulated in quadrature to each other on the color subcarrier. Quadrature modulation uses two carriers, one ninety degrees phase shifted from the other. In the receiver, quadrature detection cleanly separates the two signals. This separation is based on simple mathematics. The mathematical sine function is ninety degrees phase shifted from the mathematical cosine function. The multiplication of a sine function with a cosine function yields a pair of sine waves with angles equal to the sum and differences of the two original functions.
            F      ⁡              (        t        )              ·          sin      ⁡              (        A        )              ·          cos      ⁡              (        B        )              =            F      ⁡              (        t        )              ·          [                                    1            2                    ⁢                      sin            ⁡                          (                              A                -                B                            )                                      +                              1            2                    ⁢                      sin            ⁡                          (                              A                +                B                            )                                          ]      When A=B:
            F      ⁡              (        t        )              ·          sin      ⁡              (        A        )              ·          cos      ⁡              (        A        )              =                    F        ⁡                  (          t          )                    ·              [                                            1              2                        ⁢                          sin              ⁡                              (                0                )                                              +                                    1              2                        ⁢                          sin              ⁡                              (                                  2                  ⁢                  A                                )                                                    ]              =                  F        ⁡                  (          t          )                    ·                        1          2                ⁡                  [                      0            +                          sin              ⁡                              (                                  2                  ⁢                  A                                )                                              ]                    
If A=B then the resulting signal equals the product of the modulating signal, F(t), and the sum of the sine of zero and the sine of twice A (which equals B). The sine of zero is equal zero and if A and B are the same frequency, the result is a sine function at twice the frequency. Simple filters easily separate the baseband frequencies. All that is left is zero, after the double frequency sine is filtered out.
On the other hand, the multiplication of two cosine functions yields a cosine at their sum frequency and another cosine at their difference frequency.
            F      ⁡              (        t        )              ·          cos      ⁡              (        A        )              ·          cos      ⁡              (        B        )              =            F      ⁡              (        t        )              ·          [                                    1            2                    ⁢                      cos            ⁡                          (                              A                -                B                            )                                      +                              1            2                    ⁢                      cos            ⁡                          (                              A                +                B                            )                                          ]      When A=B:
            F      ⁡              (        t        )              ·          cos      ⁡              (        A        )              ·          cos      ⁡              (        A        )              =                    F        ⁡                  (          t          )                    ·              [                                            1              2                        ⁢                          cos              ⁡                              (                0                )                                              +                                    1              2                        ⁢                          cos              ⁡                              (                                  2                  ⁢                  A                                )                                                    ]              =                  F        ⁡                  (          t          )                    ·                        1          2                ⁡                  [                      1            +                          cos              ⁡                              (                                  2                  ⁢                  A                                )                                              ]                    
If A=B then the resulting signal is equal to the product of the modulating signal, F(t), and the sum of the cosine of zero and the cosine of twice A (which equals B). The cosine of zero is one and, if A and B are the same frequency, the result is a cosine function at twice the frequency. This is also easily separated from the baseband frequencies with simple filters. All that is left is half of the baseband modulating signal, F(t), after the double frequency cosine is filtered out. This process is called synchronous detection because the carrier frequency and phase of the received signal is synchronous with the locally supplied signal used to demodulate it.
The consequence of this synchronous multiplication of a cosine wave at the carrier frequency and the modulated cosine signal is a de-modulation yielding the original information signal at baseband frequencies; i.e. from zero frequency to the highest information frequency. The multiplication of two sine functions also results in demodulation of the information contained on the sine carrier. In this way the quadrature signals are separately detected without interfering with each other.
Compromise “Compatible” Stereo Television Sound
Television sound is frequency modulated on a separate carrier that is a fixed 4.5 MHz above the visual carrier. When stereo sound was added to the television system, the requirement of “compatibility” was again enforced to avoid the chaos that might have resulted from obsoleting the existing sound system. Just as with “compatible color”, there were compromises to monaural receivers when stereo sound was added. But the net benefit to consumers was considered to be positive. The marketplace gave its approval to both “compatible color” and “compatible stereo sound”.
Stereo sound is implemented by first creating a spectrum that includes the sum of the left and right sound channels at baseband. The difference of the left and right channels are double sideband, suppressed carrier modulated onto a carrier at twice the horizontal scan frequency (2×15,734=31,468 Hz). A limited bandwidth (10 kHz) monaural Second Aural Program (“SAP”) channel is frequency modulated onto a carrier at five times the horizontal scan frequency. The SAP channel is intended for second language or other such purposes. A very narrow bandwidth (3.4 kHz) “Professional Channel” is frequency modulated onto a carrier at six and a half times horizontal scan frequency. It is used for television plant intercommunications. This entire complex spectrum is then frequency modulated onto the 4.5 MHz carrier. The relationship between the Visual and aural carriers is tightly controlled since nearly all television receivers depend on this relationship. The visual carrier is used as the local oscillator to bring the sound spectrum down to baseband. This technique is called the “intercarrier sound” method of TV receiver design. Since the final modulation process is that of frequency modulation, the TV receiver uses a “limiter” circuit to strip off any amplitude modulation. The TV receiver then becomes insensitive to any amplitude modulation.
Early Analog Attempts at “Compatible” Advanced Television
As NTSC television approached its fiftieth anniversary, color television receivers became a commodity. Low cost receivers which provide excellent pictures and important basic features such as remote control, stereo sound, captioning for the hearing impaired sell for less than $10 per screen size inch. With only around 100 million television households in the U.S., there are in excess of 250 million television receivers and 150 million VCRs. In addition, about 25 million color new television receivers and around 15 million new VCRs are sold each year. If the average television receiver is a 19″ model, it's approximately 15 inch wide screen will be contained in a cabinet about 18″ wide. All of the U.S. TVs set side by side would stretch 71,100 miles, several times around the earth. And 7,100 miles worth of new sets are sold in the US each year—more than enough to go coast to coast a couple of times! The market is saturated and the industry has more production capacity than the market needs. A new product is desperately needed for survival of the industry. Japan began the search for a new service which would require new products for the living room. Japan launched the development of High Definition Television (HDTV) more than twenty years ago and spent over a billion dollars in pursuit of that goal.
Broadcasters also had difficulties. They were faced with a continuing loss of spectrum to the communications industry. Their once 83 channel universe was cut to 69 to give spectrum to cellular phone and mobile communications. Not satisfied, the communications industry began demanding even more spectrum. In response, broadcasters insisted that they need that spectrum for expansion to HDTV. Without the demands of the broadcasters, the spectrum would have gone to communications.
While the broadcasters wanted HDTV and had the political power to use it to preserve spectrum, the consumer electronics industry desperately needed HDTV.
The HDTV system developed in Japan is called MUSE for Multiple Sub-Nyquist Sampling Encoding. MUSE is in operation in Japan and consumer television receivers are commercially available. While MUSE is a technological marvel, it requires more than 6 MHz of bandwidth. The FCC put down more stringent requirements. The FCC required that the HDTV signal: a) fit in 6 MHz, b) be compatible with NTSC, and c) not cause undue interference with the NTSC service. At first all of these requirements seemed impossible. In the end, success was obtained on two out of three criteria. The only failure was compatibility.
The first approaches to satisfying the FCC requirements were based on retaining the NTSC signal, adding a supplementary 6 MHz signal, and adding in-band helper signals to the NTSC. This automatically satisfied the compatibility requirement and had the further advantage that the helper signals in the NTSC channel could be used to enhance the reception on new receivers. These helper signals increased the width of the picture to a 16×9 aspect ratio from the NTSC 4×3 shape. Increased resolution was also provided. It was expect that an intermediate product, called Improved Definition Television (IDTV) would fill the gap between NTSC and expensive HDTV products allowing a more rational transition. The well-off and the eager early adopters could purchase HDTV while the less well-to-do could improve their reception with compatible IDTV receivers until the cost of HDTV came down sufficiently to be widely affordable.
A number of patents and papers have discussed using a quadrature carrier as a means of carrying additional analog and even digital information in a television signal. These approaches have not achieved commercial application because of practical deficiencies and the subsequent rush to digital HDTV. The objectives of most of these approaches has been to carry supplementary information to enhance an ordinary television signal yielding an IDTV system. In some cases, these approaches are part of a High Definition Television, HDTV, system.
U.S. Pat. No. 4,882,614 filed Jul. 7, 1987, issued Nov. 21, 1989 and titled Multiplex Signal Processing Apparatus, discusses a multiplex signal processing apparatus comprising of a second amplitude modulator for modulating a second carrier which has the same frequency but differs in phase by ninety degrees from the first carrier. The second carrier is modulated by an auxiliary signal to obtain a double sideband amplitude-modulated multiplex signal. An inverse Nyquist filter is utilized for preconditioning the signal so that it becomes a double sideband signal when passed through the receiver's Nyquist filter. A multiplex signal processor at a receiver has a synchronous detector and a quadrature distortion eliminating filter for demodulating the main and multiplex signals from the received multiplexed signal. A normal synchronous receiver will produce a conventional television signal without distortion (crosstalk) caused by the quadrature auxiliary signal.
The present invention differs from the invention of U.S. Pat. No. 4,882,614 in several significant ways. Firstly, the present invention does not depend on the use of a synchronous detector in the receiver. The response of the receiver to the envelope of the amplitude modulated signal is abated using the techniques of this invention.
Secondly, the present invention does not use an Inverse Nyquist filter at the signal source. Instead it uses a Nyquist filter and a spectrum processing means to predistort the signal. This is important because the characteristic shape of a Nyquist filter is not defined. Rather, a Nyquist filter is one which has an anti-symmetric characteristic around its Nyquist frequency. This characteristic may be linear, but it does not have to be. An infinite number of possible characteristics can satisfy the Nyquist criterion. By using a Nyquist filter in the signal source, it becomes convenient to use a filter representative of the population of receivers exposed to the signal. This is accomplished by simply using the commercially available Nyquist filter most commonly used in those receivers. It is also possible to operate a number of representative Nyquist filters in parallel with the signal split between them in proportion to their presence in the population of receivers. The combined signal would then be optimized for the population of receivers exposed to the signal. This can vary from market to market and from time to time as the population of receivers changes.
Thirdly, the receiver of the present invention does not use a quadrature distortion eliminating filter. Such a filter may introduce distorting phase shifts in the received data signal causing difficulty in achieving the maximum data rates possible. Instead, the present invention filters the interfering video signal with an aggressive filter and then subtracts the distorting video signal from the received signal to leave just the auxiliary signal. In this manner, distortions introduced by a filter in the auxiliary signal path are avoided.
In the specific case of U.S. Pat. No. 4,985,769 filed Mar. 23, 1988, issued Jan. 15, 1991, and also titled “Multiplex TV Signal Processing Apparatus”, the patent's primary objective is to compatibly add side panels to an NTSC signal to make it wide screen. The side panel information is broken into two parts, low frequency and high frequency. The low frequency portion exists only in the time period of the side panels. It is time compressed which raises its frequency content up to full luminance bandwidth. It is then inserted into small time slices right after chroma burst and just before horizontal sync pulses. The spectrum of this signal has the D.C. component of the side panels. This is called the time multiplexed signal. The high frequency portion has more bandwidth than can fit into the quadrature channel created by U.S. Pat. No. 4,985,769. Since the signal exists only during the time period of the side panels, it can be stretched in time. This time stretching lowers the frequency content so that it fits into the available bandwidth of the quadrature channel created by U.S. Pat. No. 4,985,769.
U.S. Pat. No. 5,036,386 filed Jul. 19, 1989, issued Jul. 30, 1991 and titled Television Signal Processing Apparatus, recognizes that the quadrature channel has interference but assigns a Vertical-Temporal, V-T, component to it so that the correlation between the video and interference is such that it is rendered less visible in a conventional TV receiver. This patent recognizes that interference in ordinary receivers can be detected in practice because of the imperfectness of the characteristics of filters at the receiver and transmitter.
The term “transmitter” is used in this document as a generic device which modulates a signal for transmission through any medium. It includes broadcast transmitters which are normally connected to antennas and relatively low power modulators used in cable systems and other media connected to cable, wire, fiber optics or other media.
Digital Signals
The advantages of digital signals include: (1) the ability to completely regenerate the signal and prevent the accumulation of noise and distortion; and (2) the ability to apply computational techniques for multiple purposes. Included applications of computational techniques are error detection and correction and redundancy reduction. The human sensory system for images and sound is analog. Images and sounds start out as analog signals. To be enjoyed by humans, the signals must eventually be displayed as analog signals for eyes and ears to enjoy. Unfortunately, as signals are transmitted over long distances, they encounter noise, distortion, and interfering signals which degrade the quality of the images and sounds and eventually make them first unpleasant to the human ear or eye and then unusable. If the analog signals are converted to digital signals, a negligible amount of noise is introduced in the conversion process, but all subsequent degradation of the signal can be avoided using practical and well understood techniques.
To convert an analog signal into a digital signal, the analog signal must first be sampled in time. The information science theorist, H. Nyquist, proved that if a signal is sampled at a frequency at least twice the maximum signal frequency it contains, the signal can be perfectly recovered with no loss of information. Sampled signals are still analog because they can take on any value. They are just time quantized. If each time sample's strength is then measured and the resulting measurement represented by a number of limited precision, the sampled analog signal has been converted into a sequence of data. Limited precision numbers have a fixed number of decimal places. The uncertainty in precision of the number is determined by the value of its last decimal place. Thus, the information to be transmitted is no longer the original analog signal or its time sampled version (which can take on any value), but rather another signal that conveys the limited precision numbers describing the strength of the original signal samples. The representation of the signal by a limited precision number introduces an error which can be considered to be a degree of noise, called quantization noise. The amount of quantization noise can be made arbitrarily small by using arbitrarily higher precision numbers, but it can never be reduced to zero. A major advantage of the data signal approach is that techniques exist for preventing any further degradation of the signal.
The limited precision numbers used to represent the sampled analog signal may have a variety of forms. Most individuals are accustomed to using a number system based on the value ten. That is, numbers commonly used in human transactions utilize the ten numeric symbols: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. This is called a base ten or decimal system. A further characteristic of the base ten system is that the value of a number is determined by the symbol used and its place in the string of numbers. The right most place has only the symbol value. The next place to the left has the symbol value times ten. The next place to the left has the symbol value times one hundred, and so on. Thus the number 543 has the value three plus four times ten plus five times one hundred.
A similar number system can be created using just two symbols, 0 and 1. Such a system is called base 2 or “binary” because it has only two fundamental symbols. In such a system, higher values are represented by the value assigned to the location of the symbol in the number. Again, the right most location has the symbol value times one. The location to the left has the symbol value multiplied times two, or double the place value of the location to its right. The next location to the left has the symbol value multiplied times four, or double the place value of the location to its right. The next location to the left has the symbol value multiplied times eight, or double the place value of the location to its right. The process continues with each new place having twice the value as the last. So the binary number 101 is valued, starting from the right hand side, as one times one plus zero times two plus one times four. The total is five. This same process can be used to determine the value of any binary number.
The advantage of binary numbers is that they can be represented by simple and inexpensive circuits and the impact of electrical noise and distortion can be minimized or even eliminated by simple and cost-effective design methods. A circuit element, such as a transistor, which processes an analog signal must faithfully reproduce all values of the signal and add a minimum of distortion and noise. If many circuit elements process an analog signal, their individual contributions of noise and distortion accumulate causing signal degradation. A binary circuit, on the other hand, can have two well defined states, “on” and “off”, which are easily distinguishable. The “on” state may represent the binary number “1” while the “off” state can represent the binary number “0”. (The opposite choice is equally valid). The important point is that if the circuit element is mostly “off”, but not completely “off”, it will not be confused with the “on” state. Similarly, if the circuit element is mostly “on”, but not completely “on”, it will not be confused with the “off” state. Thus, imperfect performance of the circuit can still faithfully represent the binary values. Only when the “on” state approaches half of the assigned value or the “off” state is almost half way to the “on” condition can confusion result. If this degree of deficient performance is avoided, the two states can be discriminated and the signal perfectly resolved. If, as the signal is transmitted, it suffers some noise and distortion degradation, it can still be perfectly recovered as long as the two states, the “on” state representing a binary “1,” and the “off” state representing a binary “0”, can be reliably discriminated. Eventually, sufficient noise and distortion is introduced so that the two states become confused. If the system is designed so that the signal is regenerated prior to this destructive level of degradation, a fresh binary signal can be substituted for the degraded signal and all of the damage caused by noise and distortion can be completely removed. This process can be repeated an arbitrary number of times allowing error free communications over arbitrarily long distances. This is something that cannot be accomplished with analog signals.
A further advantage of digital signals is the small size and expense of modern transistors. Gordon Moore, one of the founders of Intel Corporation, observed that approximately every twelve to eighteen months, the number of digital transistors that may be stored on a single integrated circuit doubles. Alternatively, the cost of a given number of digital transistors approximately halves during that same time period. This process has been continuing for decades and appears likely to continue for some time to come. As an example of this phenomena, the first personal computers introduced in the early 1980s used an Intel brand Integrated Circuit, (“IC”), that included thirty thousand digital transistors. The Pentium computer ICs of the mid 1990s have over five million digital transistors. Tens of millions of digital transistors can be expected in consumer products at affordable prices by the end of the millennium. The same experience has not been enjoyed by analog circuits because they must faithfully process the infinite range of values of analog signals. That severe constraint has prevented analog circuits from progressing as fast or as far in complexity and cost reduction.
Yet a further advantage of digital signals and circuits is that they can be mathematically manipulated in a very complex fashion thus, simplifying methods for determining if transmission errors have occurred and how to correct such errors. Note that there are only two possible types of errors. A binary “1” symbol may be damaged and converted into a binary “0” symbol or a binary “0” symbol may be damaged and converted into a binary “1” symbol. No other alternatives exist in a binary system. As an example, a common method of error detection is to group binary symbols into clusters of seven and to append an eighth symbol depending on whether the previous seven symbols have an even or an odd number of “1” symbols. If the appended symbol produces an even number of “1” symbols in each group of eight symbols, then a single transmission error will result in an odd number of “1” symbols. Note that if two errors occur, a much less likely event, the system will be fooled and think that no error has occurred. However, if three errors occur, the damage will again be detected. The ability to detect certain error conditions is obtained at the price of an appended symbol that takes up transmission time and requires additional circuits to process at both the transmission and receiving ends. More complex schemes, called data detection and correction algorithms, can detect multiple errors and even determine the correct signal. These more complex methods increase the amount of additional, non-data symbols and are said to have increased “overhead.” Also, additional processing is required at both the sending and receiving ends of the transmission path.
Still another advantage of digital signals is that they are amenable to methods of compression that reduce the redundancy in the information and allow more information to be transmitted per unit time. A further benefit of compressed signals is that they require less memory for storage. One example of data compression is the technique of “Run Length Coding.” If a data signal contains a “run” of the same symbol, a coded message can indicate the length of the run with fewer symbols than simply transmitting the basic symbols themselves. For example, if the signal includes thirty “0” symbols, fewer than thirty symbols are required to code that fact. Another example is the use of special symbol tables defined for the information to be transmitted. Information groups with a high frequency of occurrence are assigned short digital codes and information groups with a low frequency of occurrence are assigned the remaining longer codes. The Morse code is an example of this technique. The letter “e” is the most frequently used letter in the English language. It is assigned the shortest Mores Code, the “dot”. Numbers and punctuation occur much less frequently and so are relegated to the longer series of “dots” and “dashes”. D. A. Huffman developed a method of creating such compression codes. “A Method for the Construction of Minimum Redundancy Codes” Proc. IRE vol. 40 September 1952 pp 1098–1101” which is incorporated by reference herein.
The computational nature of digital signals makes it possible to implement a great deal of processing in software on more general-purpose processors. The degree of processing can be very complex. In addition, much of the processing can be assisted with dedicated digital circuits.
Like analog signals, when binary signals are to be transmitted at radio (or television) frequencies, they must be modulated onto a carrier. The simplest modulation method is to amplitude modulate the logic levels onto the carrier with two different strengths. Then, at the receiving end, the goal is to recover the data. If, as is usually the case, the noise and distortion is modest, but not excessive, the most important task of the data demodulator is to remove the modulation. A data extractor then converts the analog representation of data into clean logic levels. This data extraction is performed with a “slicer” and a sampling circuit driven by a synchronized clock. The slicer is a circuit that compares the input signal strength with a pre-determined voltage level called a “threshold.” If the input signal is above the threshold, it is assigned one of the two logic levels. If it is below the threshold, the signal is assigned the other logic level. The output of the slicer is once again a clean signal free of noise and distortion. However, the output is not yet data since an ambiguity exists regarding the beginning and ending of the data pulses. This ambiguity is resolved by circuits that sample the logic levels at precisely the correct time. The sampling results in data pulses that are suitable for further digital logic processing in the microprocessor.
While the use of two levels representing a logic “1” and a logic “0” is almost the universal method of designing digital logic circuits, it is not necessarily the only way in which this can be done. If circuit elements were found which had other numbers of very stable states, entire logic systems could be created around them. Consider an electronic element that has four natural states. With four states, two bits could be represented at any one time with the four following combinations: 00, 01, 10, and 11. Similarly, if an electronic element with eight natural states were available, it could represent three bits at any one time.
The term “multilevel” means in this document more than one level and includes a two level signal as well as a signal with more than two levels.
While the multilevel approach with more than two levels has not yet proved commercially useful in the design of logic circuits, it is extremely useful in the transmission of data. When only two levels are transmitted, one bit per symbol time is conveyed. Such a system can tolerate noise levels almost equal to half the difference between the strength of the signal representing logic “1” and the strength of the signal representing logic “0”. In the case of systems such as Teletext, where the main signal is analog television, the noise level must be constrained to modest levels that do not approach half the difference between the strength of the signal representing logic “1” and the strength of the signal representing logic “0”. That amount of noise would result in an unacceptable analog video picture. Because the noise is much less, more levels can be accommodated. In the data receiver, after demodulation, three “level slicers” equally spaced between four levels, would support four levels of signal. Four levels of signal would transmit two bits of data simultaneously at each symbol time. Similarly, seven level slicers equally spaced between eight signal levels would support three simultaneous bits of data. This technique greatly speeds up data transmission at a modest increase in equipment complexity. At the receiver, the multilevel data with more than two levels is converted back to two level data using level slicers and logic circuits. This is necessary since the succeeding logic circuits and microcomputers of current design all deal with only two levels.
Digital Television
After an extended search for a “compatible” method of creating high definition television (“HDTV”), it became clear that all methods proposed used the original NTSC signal plus in-band and an out-of-band “helper signals”. All of the available resources were required to create the compatible signal and two 6 MHz bands were consumed.
Zenith Electronics Corporation broke the mold by proposing a hybrid system which transmitted the high frequencies of the image in analog form and the lower frequencies in a digitized form. This hybrid approach seemed to use the best of both worlds. It recognized that most of the energy in an NTSC signal is in its low frequencies which include the synchronization pulses. By digitizing the low frequencies, their major power consumption was eliminated. Yet the burden on the digital circuits was relaxed because only relatively low frequencies were processed. The high frequencies remained analog and contributed little to the power requirements. The lower data rate digital signals might also be less susceptible to multipath, while the higher frequency analog signals were less effected by noise. The remaining problem is that this approach was no longer “compatible” with existing NTSC receivers. This problem was solved by allowing the concept of “compatibility” to include the technique of “simulcasting.” That is, both the hybrid signal and the NTSC signal would carry the same programming, at two different resolutions. This compromise would keep the owners of older receivers from becoming disenfranchised. And since no system that puts both NTSC and HDTV into the same 6 MHz was proposed, two 6 MHz channels would still be required. This approach had one major advantage. If the time ever came when all of the NTSC receivers were old and new ones were no longer produced, the NTSC channel could be reallocated to other purposes. Even before that, the requirement for simulcasting could be relaxed based on policy rather than technological constraints. By this step-by-step process, “compatibility” was abandoned for the first time in television. (The non-compatible CBS color system, while temporarily the official system in the U.S., did not achieve commercial success before it was replaced with the compatible color system.)
Shortly thereafter, General Instrument Corporation proposed an all-digital solution. Quickly, most of the serious proponents (with the exception of the Japanese MUSE system) converted to all-digital. The committee charged with selecting a winner, found that it could not. The technical issues were too complex and the political issues were overwhelming. At the time a decision was to be made, all of the proposed systems created unacceptable pictures. The result was a decision to score all of the systems as acceptable under the condition that a “Grand Alliance” be formed allowing the proponents themselves to decide upon a single system. Thus the political battles could occur behind close doors under the guise of selecting the “best” of each proponent for a single proposal to the FCC.
Recent developments in the television industries have focused upon the transmission of HDTV, which requires a substantial increase in transmitted information and, therefore, could greatly expand the required Visual signal bandwidth. Nevertheless, the television industry has created standardized “tool kits” for processing images and maximizing the efficiency of transmission and storage of the resulting digital realizations. An important system for accomplishing this is known as the Moving Picture Experts Group (“MPEG”) standard. MPEG consists of a collection of techniques that can be selected depending on the nature of the application. This progress in the area of digital TV bandwidth compression has resulted in a national standard being selected by the FCC in December, 1996. Using this standard, a single HDTV channel can now be transmitted within the analog Broadcast TV channel assignment of 6 MHz rather than tens of megahertz once thought necessary. In the case of cable's well-behaved spectrum, double the data transmission rate is possible. Two HDTV signals can be carried in 6 MHz.
It didn't take very long for the realization to hit home that if HDTV signals could be compressed by this tremendous ratio, NTSC signals be compressed as well. Multiple NTSC signals could then be squeezed into the same 6 MHz that could carry one HDTV signal or one NTSC analog signal. These multiple signals took on the name “Standard Definition (digital) TV” (SDTV).
Actually, SDTV is a misnomer. Because of compression, the bandwidth of the original baseband signal as well as the received and reconstructed signal could exceed the 4.2 MHz limitations of the NTSC channel. Additionally, the chroma resolution can be substantially increased and nearly all of the NTSC artifacts can be eliminated. Really nice pictures are possible. Alternatively, SDTV does not have to have as much resolution as NTSC. It is possible to reduce resolution and increase the number of programs carried in 6 MHz.
The term “program” is used in this document in the general sense to include any information which may be in need of transportation from one location to another. It includes but is not limited to television programming and includes computer programs, digital information, Internet information, and any other signals which can be conveyed from one user to another using the technology described.
Using techniques that share capacity between multiple programs, it is possible to apply the “statistical multiplexing” used in the telephone industry for decades to further increase capacity. The trade-off between quantity and quality offers bewildering choices. The American consumer has long voted for quantity over quality.
This same technology which makes HDTV in 6 MHz possible allows multiple standard definition digital signals to be stuffed into 6 MHz. Movies have several advantages over video in this regard. Movies have twenty four frames per second versus video's thirty. This distinction alone is a twenty percent reduction in data requirements. Movies have the further significant advantage in that they can be processed iteratively. That is, the movie is run through the processor several times with adjustment of the processor made to minimize the creation of artifacts on a scene by scene basis. Very good results have been obtained with movies at data rates of 3.0 Mb/s. Quite acceptable results have been seen at 1.5 Mb/s. When compared to the video obtained from a commercially recorded VHS cassette, the digital results have some advantages. Since the HDTV transmission rate is around 19 Mb/s (in 6 MHz), six 3.0 Mb/s movies can be carried in the same spectrum. At 1.5 Mb/s, double that number, twelve, is possible. Since cable has a more controlled spectrum, it can approximately further double these numbers leading to perhaps twenty-four movies in 6 MHz. This result is even more practical in systems that use statistical multiplexing.
The development of HDTV and its acceptance as a future broadcast standard has led to the need for a transition period between broadcasting the present analog TV to broadcasting compressed digital HDTV.
It is expected that the transmission of standard analog NTSC will continue for many years before a complete transition to digital high definition occurs. Some believe that this transition may take a very long time. Others hold that it may never be completed because of the vast installed base of analog receivers. The availability of a technique allowing simultaneous, non-interfering transmission of digitized NTSC-resolution signal(s) within the same channel as an analog NTSC signal would result in a two-fold (or more) expansion of channel capacity in the existing broadcast frequency assignments. If more efficient means of bandwidth compression emerge, the simultaneous transmission of HDTV and analog NTSC is an attractive possibility.
Prior Art Methods of Adding Data to Analog Television
Sub-Visual Techniques: Under-utilized portions of the NTSC spectrum can be employed to “hide” data. In many cases, the process of hiding the data is incomplete and results in artifacts under certain conditions. In other cases, the preparation of the NTSC signal to more effectively hide data itself, reduces video quality. Thus, the challenge is to both hide the data and not impair video quality while retaining signal robustness and the potential for an economic implementation.
The National Data Broadcasting Committee (“NDBC”) was formed in 1993 to establish a single standard for data transmission in visual. The NDBC issued a Request For Proposals (“RFP”) and narrowed down the selection process to two contenders: WavePhore and Digideck. Laboratory tests were conducted by the Advanced Television Test Center (“ATTC” in Alexandria, Va. in December, 1994. In April, 1995, the NDBC selected Digideck for field testing. In June, WavePhore convinced the committee to re-test their system after WavePhore made improvements based on the results of the lab tests.
Meanwhile, the FCC issued a Notice of Proposed Rulemaking (“NPRM”) in April, 1995. On Jun. 28, 1996, the FCC approved digital data transmission in the visual portion of broadcast television transmission in its Report & Order (“R & O”), “Digital Data Transmission Within the Video Portion of Television Broadcast Station Transmissions”, MM docket No. 95-42 which is incorporated by reference herein. This R & O amends FCC rules to allow ancillary data within the visual portion of the NTSC signal in four formats. Two of the formats, by Yes! Entertainment Corporation and A. C. Nielsen Co. place low data rate signals in the overscan region of the picture. The other two systems, Digideck and WavePhore, embed the digital signal into the visual signal. Both Digideck and WavePhore participate in the NDBC, sponsored by the National Association of Broadcasters (“NAB”) and the Consumer Electronics Manufacturers Association (“CEMA”). NDBC has conducted field tests of these systems in Washington, D.C. on WETA, channel 26 and WJLA, channel 7. This same R & O encouraged others to invent ways of embedding data in the analog visual signal.
WavePhore: WavePhore utilizes a teletext-like system in lines 10 through 20 in each field for a data speed of up to 150 kb/s. WavePhore added substantial error detection and protection bits to its structure to protect against multipath and other transmission problems.
The WavePhore system begins by reducing visual luminance and chrominance bandwidths. The “luminance” is reduced from its theoretical value of 4.2 MHz to 3.9 MHz and the upper sideband of the color signal is reduced by approximately 300 kHz. It is then possible to insert a data signal in this region at a carrier frequency of approximately 4.197 MHz above the visual carrier and a strength approximately 20 dB above the noise floor of the visual system. The data is synchronous with the visual carrier and thus with the horizontal line frequency. As an odd multiple of one-quarter the horizontal scan frequency, the data interleaves between the luminance and chrominance bundles of spectral energy. Data is not sent during the vertical and horizontal blanking intervals. Thirty bits of data are sent per video line. There are 240 available lines per field (not counting the VBI during which the signal is blanked). This yields a raw data rate of 435.6 kb/s. After error correction coding, the raw date rate is reduced to approximately the T1 rate divided by four or 384 kb/s. WavePhore calls their system TVT1/4 because the resulting data rate is equal to one-quarter the telephone T1 data rate.
WavePhore shuffles the data before applying bi-phase modulation and filtering out the lower sideband. Shuffling the data reduces its visibility in the video. An adaptive equalizer is used in the receiver. A major advantage of the WavePhore approach is that once inserted into the video, it can be conveyed through the visual path without giving it further attention. The WavePhore VBI system and the WavePhore sub-visual system can be combined to provide over 500 kb/s.
There is some degradation of pictures using the WavePhore system. Nevertheless, it appears that the FCC is willing to let the broadcaster determine the choices of his individual marketplace and to respond to those choices.
Digideck: The Digideck system adds a Differential Quadrature Phase Shift Key (“DQPSK”) signal carrying about 500 kb/s placed one MHz below the visual carrier. In this regard, it is similar to the European NICAM system for adding digital audio to analog television broadcasts. This modulation places the new carrier in the VSB region of the signal. To accommodate this, the lower VSB slope is increased. Rather than starting at the traditional 750 kHz below picture carrier, in the Digideck system, it starts 500 kHz and drops more rapidly. The carrier is about 36 dB below peak power and has a raw capacity of 700 kb/s. Forward error correction and other overhead burdens reduce the data capacity to around 500 kb/s. Digideck calls the new carrier the “D-Channel”. The data signal is clocked synchronously to the television signal for ease of recovery and for better hiding in the video.
The Digideck receiver also depends on an adaptive equalizer. A consequence of the D-Channel is that it must be inserted at the transmitter site and brought there by an alternate path. Like the WavePhore system, Digideck introduces some artifacts. A marketplace approach will allow the broadcaster to determine acceptability.
Overscan Techniques: Other systems have different drawbacks. The Yes! Entertainment Corporation's system introduces a pulse in the video between 9.1 and 10.36 microseconds following the start of the horizontal synchronization pulse. The data rate is very low, about 14 kb/s. Its application is to deliver audio to a talking toy teddy bear. A. C. Nielsen uses line 22 of one field of the video for transmitting a program source identification. This ID is used to measure the viewing population for statistical purposes. A fifth system, by En Technology was denied permission at the time of the R & O. This system allowed data to extend from the VBI into all areas of the picture with the image being constrained to a variable size box surrounded by the “snow” caused by the data. This system was judged too intrusive.
Quadrature Data: As discussed earlier, the patents and articles which added supplementary analog information to the television signal in a quadrature channel also mentioned that digital information could also be conveyed in this manner. While most of the techniques described in this section for carrying data have been proposed to the FCC and approved for commercial use, the quadrature carrier approach has not been proposed or commercialized.
Data on the Aural Signal: Amplitude modulation of the aural carrier has been used in the cable industry for decades for the implementation of conditional access to premium programming. Initially, this took the form of a sinusoidal amplitude modulation of the frequency modulated aural carrier which could be detected and used to remove a complimentary amplitude modulation of the video waveform. That amplitude modulation of the video carrier suppressed the synchronization pulses of the television signal, preventing the television receiver from synchronizing its horizontal and sometimes its vertical scan rates and thereby scrambled the signal. The amplitude modulation on the aural carrier provided the key to undoing that amplitude modulation of the video signal and restoring it to substantially its original form. This technique was later extended to the conveyance of data in the form of binary levels of very low rate. Addressing data allowed the individual control of set top boxes so that each subscriber could be individually controlled. As the public became more skilled in defeating these systems and stealing the service, the suppliers to the cable industry adopted more sophisticated methods for encrypting the data and protecting the service from theft. However, these signals were in all known cases limited to binary signals of low data rate. Multiple level signals of high data rate are not known to have been implemented.